Development of external genitalia requires tight coordination of proximodistal outgrowth, three-dimensional patterning and tubular morphogenesis. Congenital malformations of the penis arise when these processes are disrupted. The most common penile anomaly is hypospadias, which is characterized by failure of urethral tube closure, and is often accompanied by abnormal dorsal-ventral patterning. The latter can result in penile curvature (chordee) or ventral penile agenesis. These children have mislocalized, multiple or oversized urethral openings, and males with severe hypospadias are born with ambiguous genitalia. In the US, the frequency of hypospadias doubled, without explanation, from 1968 to 1993, and now affects 1:250 live births. Despite this, there is a relatively poor understanding of the cell types that give rise to, and the molecular mechanisms that control morphogenesis of, external genitalia. Identifying urethral progenitor cells will be essential for understanding how urethral tube defects arise developmentally. Tissue engineering &biological repair of urologic organs also will require knowledge of the cell types involved in genitourinary organogenesis. Our goals are therefore, (a) to generate a detailed spatial &temporal map of the cell lineage that forms the penile urethra, and (b) to define the mechanisms by which Sonic hedgehog (Shh), a key signaling molecule produced by urethral plate cells, patterns the dorso-ventral axis of the penis. We will achieve these aims using two new transgenic mouse models to genetically label endodermal cells in a spatially- and temporally-controlled manner, and to follow their fates during penile development. These mice also allow us to target transgenes to urethral epithelial cells and to control the timing of gene inactivation. We will take a genetic approach to testing the following hypotheses;(I) The distal (glandar) urethra is derived from Shh-expressing endodermal cells;(II) Different regions of the urethra arise from cells that express Shh at different developmental time points;and (III) Shh expression has distinct early and late roles in patterning the genitalia. These studies will provide the first insight into the origin of the penile urethra by generating a comprehensive map of cell lineage over time. Revealing how dorso-ventral pattern is regulated in the genital tubercle will improve our understanding of the developmental basis of hypospadias. The results will be pertinent to tissue engineering of urologic organs and cell-selective gene targeting in heterogeneous tissues.